Abstract: The present invention relates to the use of peroxometallic compounds, such peroxovanadium compounds, to prevent angiogenesis, restenosis and the production of endothelins, as immunomodulators and as antitumorigenic agents. Peroxometallic compounds are preferred since they are more potent, and less toxic, than their “oxo” counterparts, as exemplified by specific peroxovanadium compounds. Anti-angiogenic activity was verified in vitro against human umbilical vascular endothelial cells (HUVECs) as well as ex ovo using the chicken chorioallantoic assay membrane and in the rat aortic ring model and a Matrigel assay in vivo. Peroxovanadium compounds also decrease basal levels and inhibit the increase in plasma endothelins occurring following insulin induction in rats.